bitlocker external keys

hi,
heads up, question not related to hashcat.

had a question and hopefully can some one answer that to me.

recently i have checked a thumb drive (DOK) which is encrypted with bitlocker.

when checking the numeral password and the external keys
with this command "manage-bde -protectors X: -get"

i have received 4 different External Key.

i decide to encrypt one of my drives to see if the result is the same, instead of receiving 4 external keys i received only one external key for my drive.

my question is, what does this External Key means? 

and why they are 4 in that DOK and only one in mine.

thanks.
alot